Typically, magneto-resistive (XMR) sensors include a supporting magnet and one or more XMR sensor elements for measuring a magnetic field. The supporting magnet and the XMR sensor elements are in a fixed position relative to each other. The XMR sensor elements do not usually operate in their saturation range and the supporting magnet provides a back bias magnetic field that is superimposed on the XMR sensor elements. The supporting magnet acts as a source of the magnetic field and for some types of XMR sensor elements, such as anisotropic magneto-resistive (AMR) sensor elements, the magnetic field stabilizes the transfer characteristic of the XMR sensor elements. As the position of a detected object changes relative to the source of the magnetic field, the magnetic field produces a proportional voltage signal in the XMR sensor elements. Suitable XMR sensor elements include AMR sensor elements, giant magneto-resistive (GMR) sensor elements, tunneling magneto-resistive (TMR) sensor elements, and colossal magneto-resistive (CMR) sensor elements. XMR sensors can be used as proximity sensors, motion sensors, position sensors, or speed sensors.
Often, in speed sensors, a permanent magnet is attached to a magnetic field sensor that includes multiple XMR sensor elements. The magnetic field sensor is placed in front of a toothed magnetically permeable wheel or disk. As the disk rotates, the teeth pass in front of the magnetic field sensor and generate small field variations in the magnetic field. These small field variations are detected by the XMR sensor elements and include information about rotational speed and angular position of the rotating disk.
However, diverging magnetic field lines of the back bias magnetic field may provide components that affect the XMR sensor elements. Even if the toothed wheel or gear wheel is symmetrically aligned with the XMR sensor elements, where a tooth center or gap center is directly between the XMR sensor elements and in the middle of the back bias magnetic field, the diverging magnetic field lines may drive the XMR sensor elements into saturation and render the XMR sensor elements useless for detecting variations in the magnetic field.
For these and other reasons there is a need for the present invention.